The present invention relates to image processing devices for processing image data such that quasi-grayscale processes carried out for individual channels do not result in blotched images, image forming devices incorporating the same, and storage media for storing programs for the image processing devices to carry out the processes.
Color ink printers, color ink copying machines, color laser printers, color laser copying machines, and other image forming devices that incorporate color printer devices often receive input image data, such as multitone RGB data, that represents a different number of tones from the number of tones that the color printer devices can actually print; even when the input image data represents the same number of tones as that actually printable by the devices, it often still represents a different set of color densities from the set of color densities that the color printer devices can actually print. Therefore, in these image forming devices, the data is converted to multiple values, e.g., binarized, for each ink or toner channel so as to determine whether a dot will be formed in ink or toner of that color; thus, the image forming devices can approximately output the image represented by the input data, by distributing dots of those color densities that are available with the color printer devices. The data may be converted to multiple values by a dither or error diffusion (error dispersion) technique. If error diffusion is employed for example, original image data and errors expressed in multiple values are dispersed to surrounding pixels.
However, if a quasi-grayscale process is carried out separately for each channel, the quasi-grayscale processes are likely to cause a dot to be undesirably printed on top of another. This results in the formation of black spots, tarnishing the image with dark blotches and rendering the image appear less saturated.
Specifically, for example, when data representing a high saturation, such as an image of a lemon, is provided for input, dots are not formed faithfully to the input data due to the quasi-grayscale processes carried out for various channels. As a result, unwanted black spots are formed as shown in FIG. 48 for example even though the input data contains no black spots. FIG. 48 shows the output image at a magnification of 60, depicting different colors using a hatching scheme. To the user, the black spots appear as dark blotches against skin color and highly bright or saturated color in background. The formation of black spots reduces saturated components and causes the image to appear less saturated.
To solve these problems, for example, Japanese Laid-Open Patent Application No. 10-81026/1998 (Tokukaihei 10-81026; published on Mar. 31, 1998) discloses, subsequent to the formation of dots for the M channel by a dither technique, the formation of dots in the C channel for hues closely related to the M channel while correcting errors having developed from the formation of the M channel by adding the errors times weight factors to the C channel.
However, in such an arrangement, the sequence of channels in which the channels are processed is predetermined; less dots are likely to be formed in channels placed later in the sequence, causing dot imbalance to develop between the channels.
The present invention has objects to offer an image processing device for processing image data which causes no dot imbalance between channels and which does not allow quasi-grayscale processes carried out for individual channels to cause a blotched image, image forming devices incorporating such an image processing device, and a storage medium for storing programs for the image processing device to carry out the processes.
To achieve the above objects, an image processing device in accordance with the present invention includes:
a data output section for providing output data representative of an image constituted by pixels formed by a dot, overlapping dots, or no formation of dots at all in predetermined channels including channels for a plurality of hues;
a first comparison section for comparing a density in each of the hue channels with a predetermined threshold value for each pixel constituting a color input image; and
a dark blotch prevention section for adjusting the output data so as to reduce the density in at least one of the channels with densities higher than the threshold values if the density is higher than the threshold value at least in a predetermined number of channels for a target pixel in the comparison.
In the arrangement, data representing a target pixel and having been subjected to a quasi-grayscale process for each channel is inputted. Then, the first comparison section evaluates whether or not the input density exceeds a threshold value for each hue channel. If the density exceeds the threshold value at least in a predetermined number of channels, the dark blotch prevention section adjusts output data so as to reduce the density in at least one of those channels with threshold-value-exceeding densities. Therefore, the pixel formed according to the output data has brightness, saturation, and hue that are different from a black spot for the reduction in the densities. As a result, even when data having subjected to a quasi-grayscale process for each channel is inputted, the output data represents restrained occurrence of dark blotches due to unnecessary formation of overlapping dots.
Further, in the arrangement, the output data is adjusted after the density is evaluated in each hue channel; therefore, a better dot balance is achieved unlike in an arrangement disclosed in Japanese Laid-Open Patent Application No. 10-81026/1998 (Tokukaihei 10-81026; published on Mar. 31, 1998) where the densities are evaluated sequentially in one channel at a time.
In addition to the arrangement, preferably, the predetermined channels include cyan, magenta, and yellow channels, and the dark blotch prevention section (first quantized value substitution section 34) adjusts the output data so as to reduce the density in one of the three channels, for example, the channel with the lowest density, if the density is higher than the threshold value in all of the three channels of cyan, magenta, and yellow.
In the arrangement, the density is reduced in one of the channels when a black spot is supposed to be formed, i.e., dots are supposedly formed overlapping one another for the three channels. Therefore, the image processing device can provide output data with restrained occurrence of dark blotches and less decrease in saturation due to the quasi-grayscale processes.
As an alternative to the arrangement in which overlapping three dot formation is detected, the image processing device may further includes a brightness evaluation section for comparing brightness of the target pixel with a predetermined brightness threshold value,
the predetermined channels includes cyan, magenta, and yellow channels, and
the dark blotch prevention section, if both the density is higher than the threshold value in two or more of the three channels of cyan, magenta, and yellow, and the brightness is higher than the threshold value, selects one of the channels with densities higher than the threshold values and adjusts the output data so as to reduce the densities in the remaining channels.
In the arrangement, if the target pixel has a high brightness, only one of the channels is selected, and the density is reduced in the other channels. As a result, overlapping dot formation is interrupted when the target pixel has a brightness and the decrease in brightness due to overlapping dot formation is readily identified. As a result, the image processing device can provide output data, while restraining the occurrence of dark blotches and the decrease in saturation and brightness caused by the quasi-grayscale processes.
In order to achieve the above objects, an image processing device in accordance with the present invention includes: any one of the foregoing image processing device; and a dot formation section that can form dots for the channels overlapping one another on a printing material according to the output data from the image processing device.
In the arrangement, the dot formation section is controlled by means of the output data from the image processing device of any one of the foregoing arrangements; therefore, the image forming device can form images without losing dot balance, while restraining the occurrence of dark blotches due to unnecessary dot overlapping.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.